Your comment about "already entered the number of shares withheld to pay taxes" suggests to me that you are using, or used, the RSU step by step process as that's the ONLY place in TurboTax that asks that question. Although I guess you could be referring to entering the sales of shares sold "for taxes"? But then you also mention "the same page where I enter the corrected cost basis" and suggest that you're using the same method I recommended to mrbusto71. You can't use both methods, the RSU step by step or the "normal" 1099-B entry with basis correction, you only use one or the other.
If you are using the RSU step by step method, entering the shares withheld "for taxes" does absolutely nothing, except lower the number of share available for sale "not for taxes". That is, you get no credit for "taxes withheld" on your income tax return because you told TurboTax the number of shares withheld for taxes.
When entering the sale of these shares using the regular "1099-B" interview - where you simply transcribe numbers of your paper 1099-B into a TurboTax screen that uses the same box numbers - you only enter taxes withheld if those taxes are actually showing up on the "official" 1099-B. Taxes withheld are normally reported on the W-2.
